Organic thin film electronic devices include such things as transistor circuits, light emitting devices (e.g., OLEDs), and photo-responsive devices such as photodetectors. Typically, these devices contain one or more organic material layers positioned between electrically-conductive electrodes used to activate or read the device. Patterning of electrodes or other features over top an organic thin film is often carried out using photolithography which can be problematic in electronic device fabrication. Most organic semiconductor materials are chemically incompatible with standard photolithographic techniques.
In such devices the active organic semiconductor layer typically must be laterally patterned to define the device area. Many patterning techniques have been developed to address the unique requirements of organic semiconductors. Inkjet printing, nozzle printing, thermal vapor jet printing, and organic vapor jet printing (OVJP) are four such directed stream printing technologies for organic semiconductors and are described below.
Inkjet printing technology is based on direct patterning of substrates using solutions or inks of organic semiconductors which are pre-dissolved in solvents. The method essentially involves the ejection of a fixed quantity of an ink from a nozzle to the substrate under piezoelectric action. The ejected drop falls under the action of gravity and air resistance until it impinges on the substrate at a pre-defined well structure. The drop is then dried to remove solvent leaving the organic semiconductor in the pre-defined well forming pixel pattern. IJP technology has the potential to deliver cost-effective, large area printing of organic electronic materials with high material utilization.
Nozzle printing is also a solution (solvent-solute combination) based technology. Nozzle printing consists of a continuous extrusion of solution through a small diameter nozzle which moves at high speed to print pixel pattern. High throughput, good pixel uniformity, and material utilization make this technique attractive and cost-effective for printing organic semiconductors.
Most consumer inkjet printers are based on thermal vapor jet printing technology. This method of printing consists of rapid vaporization of inks to form a bubble by passing a pulse of current through a heating element placed in a tiny chamber of the print cartridge. Bubble formation causes a large pressure increase which propels a droplet of ink onto the substrate. The inks used in this technique are usually water-based (aqueous) and pigments or dyes are used as colorants. Print heads used in thermal vapor jet printing are cheaper to produce than print heads used in conventional inkjet technique.
Organic vapor jet printing (OVJP) enables the direct, high resolution, and rapid printing of molecular organic semiconductor devices on a variety of substrates. In this technique, organic molecules are sublimated into a hot, inert carrier gas and expanded through microscopic nozzles forming a highly collimated gas jet. The gas jet impinges on a cooled substrate to form a well-defined thin film of organic molecules whose lateral extent is governed by the nozzle diameter, nozzle-to-substrate distance, and the downstream ambient pressure. Thus, direct patterning of organic electronic materials is possible using OVJP technique which is solvent-free (compare inkjet printing and nozzle printing techniques) and has the potential to be cost-effective.